This invention relates to improvements in blind fasteners and to the joints produced therewith.
Advanced composite materials have recently been gaining increasing usage in various aerospace applications because they offer potential weight savings and increased stiffness. The anisotropic properties of composite materials present a unique opportunity to optimize certain structural designs such as elongated aircraft stringers and ribs. In order to form acceptable joints of relatively thin sheet composite materials, conventional blind fasteners have not proven to be completely acceptable. One reason is the low allowable bearing stress of composite materials, which means that the full tensile strength of the blind fastener may not be realized because of bearing failure of the composite material. Present blind fasteners have a relatively limited blind side bearing area and typically expand to approximately 1.3 times the original sleeve diameter. Another reason is that composite materials are weaker in shear, thus causing any holes to require greater edge distances and more area build up to develop full efficiency of the composite material. These materials are further constructed of a resin matrix which has a tendency to crack when holes in the material are expanded, such as by cold working or by an interference fit fastener. Rivets which swell when upset may also crack the resin matrix. Composite materials are generally fabricated in layers and can be formed to complex shapes whose surfaces are not flat. Conventional blind fasteners generally cannot form to the irregular contour of the composite material surface and tend to crush the high points of the material, reducing overall material strength.
In order to solve these problems, the present invention is directed to an improved blind fastener which has a large bearing area, does not expand the holes in respective workpieces forming a joint in which the fastener is installed and forms to the contour of the blindside workpiece surface.